Catadioptric illumination system for metrology

ABSTRACT

A catadioptric optical system operates in a wide spectral range. In an embodiment, the catadioptric optical system includes a first reflective surface positioned and configured to reflect radiation; a second reflective surface positioned and configured to reflect radiation reflected from the first reflective surface as a collimated beam, the second reflective surface having an aperture to allow transmission of radiation through the second reflective surface; and a channel structure extending from the aperture toward the first reflective surface and having an outlet, between the first reflective surface and the second reflective surface, to supply radiation to the first reflective surface.

This application is a continuation of U.S. patent application Ser. No.13/164,196, filed on Jun. 20, 2011, which application claims priorityand benefit under 35 U.S.C. §119(e) to U.S. Provisional PatentApplication No. 61/357,411, filed on Jun. 22, 2010. The content of eachof the foregoing applications is incorporated herein in its entirety byreference.

FIELD

The present invention is generally directed to optical systems, and moreparticularly to catadioptric optical systems.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate or part of a substrate. A lithographic apparatus can beused, for example, in the manufacture of flat panel displays, integratedcircuits (ICs) and other devices involving fine structures. In aconventional apparatus, a patterning device, which can be referred to asa mask or a reticle, can be used to generate a circuit patterncorresponding to an individual layer of an IC, flat panel display, orother device. This pattern can be transferred onto all or part of thesubstrate (e.g., a glass plate, a wafer, etc.), by imaging onto a layerof radiation-sensitive material (e.g., resist) provided on thesubstrate.

The patterning device can be used to generate, for example, an ICpattern. The patterning device can additionally or alternatively be usedto generate other patterns, for example a color filter pattern or amatrix of dots. Instead of a mask, the patterning device can be apatterning array that comprises an array of individually controllableelements. The pattern can be changed more quickly and for less cost insuch a system compared to a mask-based system.

After patterning the substrate, measurements and inspection aretypically performed. The measurement and inspection step typicallyserves two purposes. First, it is desirable to detect any target areaswhere the pattern in the developed resist is faulty. If a sufficientnumber of target areas are faulty, the substrate can be stripped of thepatterned resist and re-exposed, hopefully correctly, rather than makingthe fault permanent by carrying out a process step, e.g., an etch, witha faulty pattern. Second, the measurements may allow errors in thelithographic apparatus, e.g., illumination settings or exposure dose, tobe detected and corrected for in subsequent exposures.

However, many errors in the lithographic apparatus cannot easily bedetected or quantified from the patterns printed in resist. Detection ofa fault does not always lead directly to its cause. Thus, a variety ofoff-line procedures (i.e., procedures carried out in addition to normalprocessing of the substrate) for detecting and measuring errors in thelithographic apparatus are known. These may involve replacing thesubstrate with a measuring device or carrying out exposures of specialtest patterns, e.g., at a variety of different machine settings. Suchoff-line techniques take time, often a considerable amount, reducingproduction time and during which the end products of the apparatus willbe of an unknown quality until the measurement results are madeavailable. In-line measurement and inspection procedures (i.e.,procedures carried out during the normal processing of the substrate)are known.

Optical metrology techniques may be used to perform the measurements andinspection. For example, scatterometry is an optical metrology techniquethat can be used for measurements of critical dimension (CD) andoverlay. There are two main scatterometry techniques:

(1) Spectroscopic scatterometry measures the properties of scatteredradiation at a fixed angle as a function of wavelength, usually using abroadband light source, such as a xenon, deuterium, or halogen basedlight source such as a xenon arc lamp. The fixed angle can be normallyincident or obliquely incident.

(2) Angle-resolved scatterometry measures the properties of scatteredradiation at a fixed wavelength as a function of angle of incidence,usually using a laser as a single wavelength light source.

Using scatterometry the structure giving rise to a reflected spectrum isreconstructed, e.g., using real-time regression or by comparison to alibrary of patterns derived by simulation. Reconstruction involvesminimization of a cost function. Both approaches calculate thescattering of light by periodic structures. The most common technique isRigorous Coupled-Wave Analysis (RCWA), though radiation scattering canalso be calculated by other techniques, such as Finite Difference TimeDomain (FDTD) or Integral Equation techniques.

SUMMARY

Known scatterometers, however, have one or more drawbacks. For example,conventional scatterometers only detect one wavelength at a time. As aresult, spectra with more than one wavelength have to betime-multiplexed, which increases the total acquisition time taken todetect and process the spectra.

Accordingly, it is desirable to, for example, to have a metrology toolcapable of handling a wide range of wavelengths.

According to an embodiment of the present invention, there is provided ametrology tool, comprising an objective to deliver radiation to asurface and to receive radiation redirected by the surface; a detectorto receive the redirected radiation from the objective; and anillumination system to deliver the radiation for redirection to theobjective, the illumination system comprising a catadioptric opticalsystem. Use of a catadioptric optical system can facilitate a toolhandling a wide range of wavelengths.

According to an embodiment of the present invention, there is provided acatadioptric optical system, comprising: a first reflective surfacepositioned and configured to reflect radiation; a second reflectivesurface positioned and configured to reflect radiation reflected fromthe first reflective surface as a collimated beam, the second reflectivesurface having an aperture to allow transmission of radiation throughthe second reflective surface; and a channel structure extending fromthe aperture toward the first reflective surface and having an outlet,between the first reflective surface and the second reflective surface,to supply radiation to the first reflective surface.

According to an embodiment of the present invention, there is provided ametrology method, comprising: delivering radiation to a surface using anobjective; receiving radiation redirected by the surface using theobjective; detecting a parameter of the surface using the redirectedradiation from the objective; and delivering the radiation forredirection to the objective using a catadioptric optical system.

Further embodiments, features, and advantages of the present invention,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the relevant art(s) to makeand use the invention.

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention.

FIGS. 2A to 2C schematically depict the principles of angle-resolved andspectroscopic scatterometry.

FIG. 3 schematically depicts another example scatterometer.

FIG. 4 schematically depicts a sensing and alignment system including acatadioptric optical objective in accordance with an embodiment of thepresent invention.

FIGS. 5-8 schematically depict various catadioptric optical systems inaccordance with embodiments of the present invention.

FIG. 9 schematically depicts rays traversing the catadioptric opticalsystem of FIG. 8.

FIG. 10 schematically depicts a catadioptric optical system thattransforms from a very high numerical aperture to a low numericalaperture.

FIG. 11 schematically depicts an illumination branch of a scatterometeraccording to an embodiment of the invention.

FIG. 12 schematically depicts a catadioptric condenser lens according toan embodiment of the invention.

FIG. 13 schematically depicts a catadioptric condenser lens according toan embodiment of the invention.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements.

In the specification, reference to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicates that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

DETAILED DESCRIPTION

Before providing additional details of optical systems according to oneor more embodiments of the present invention, it is first helpful todescribe an example lithography environment and scatterometery system inwhich such optical systems may be used.

Example Lithography Environment

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe present invention. The apparatus of FIG. 1 comprises an illuminatorIL, a support structure MT, a substrate table WT, and a projectionsystem. As herein depicted in FIG. 1, the apparatus is of a transmissivetype (e.g., employing refractive optical elements in the projectionsystem). Alternatively, the apparatus can be of a reflective type (e.g.,employing substantially only reflective elements in the projectionsystem).

The illuminator IL is configured to condition a radiation beam B (e.g.,a beam of UV radiation as provided by a mercury arc lamp, or a beam ofDUV radiation generated by a KrF excimer laser or an ArF excimer laser).The illuminator IL may include various types of optical components, suchas refractive, reflective, diffractive, magnetic, electromagnetic,electrostatic or other types of optical components, or any combinationthereof, for directing, shaping, or controlling radiation.

The support structure (e.g., a mask table) MT is constructed to supporta patterning device (e.g., a mask) MA having a patterning device patternMP and connected to a first positioner PM configured to accuratelyposition the patterning device in accordance with certain parameters.

The substrate table (e.g., a wafer table) WT is constructed to hold asubstrate (e.g., a resist coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters.

The projection system (e.g., a refractive projection lens system) PS isconfigured to project the radiation beam B modulated with the pattern MPof the patterning device MA onto a target portion C (e.g., comprisingone or more dies) of the substrate W. The term “projection system” usedherein should be broadly interpreted as encompassing any type ofprojection system, including refractive, reflective, catadioptric,magnetic, electromagnetic and electrostatic optical systems, or anycombination thereof, as appropriate for the exposure radiation beingused, or for other factors such as the use of an immersion liquid or theuse of a vacuum. Any use of the term “projection lens” herein can beconsidered as synonymous with the more general term “projection system.”The projection system can image the pattern of the patterning device,such that the pattern is coherently formed on the substrate.Alternatively, the projection system can image secondary sources forwhich the elements of the patterning device act as shutters. In thisrespect, the projection system can comprise an array of focusingelements such as a micro lens array (known as an MLA) or a Fresnel lensarray to form the secondary sources and to image spots onto thesubstrate. The array of focusing elements (e.g., MLA) comprises at least10 focus elements, at least 100 focus elements, at least 1,000 focuselements, at least 10,000 focus elements, at least 100,000 focuselements, or at least 1,000,000 focus elements.

The support structure MT holds a patterning device MA. It holds thepatterning device MA in a manner that depends on the orientation of thepatterning device MA, the design of the lithographic apparatus, andother conditions, such as for example whether or not the patterningdevice MA is held in a vacuum environment. The support structure MT maybe a frame or a table, for example, which may be fixed or movable asrequired. The support structure MT may ensure that the patterning deviceMA is at a desired position, for example with respect to the projectionsystem PA. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to modulate thecross-section of a radiation beam (e.g., impart a radiation beam 13 witha pattern in its cross-section), such as to create a pattern in a targetportion of the substrate. The devices can be either static patterningdevices (e.g., masks or reticles) or dynamic (e.g., arrays ofprogrammable elements) patterning devices. Examples of such patterningdevices include reticles, programmable mirror arrays, laser diodearrays, light emitting diode arrays, grating light valves, and LCDarrays

It should be noted that the pattern imparted to the radiation beam B maynot exactly correspond to the desired pattern in the target portion C ofthe substrate W, for example if the pattern includes phase-shiftingfeatures or so called assist features. Similarly, the pattern eventuallygenerated on the substrate may not correspond to the pattern formed atany one instant on an array of individually controllable elements. Thiscan be the case in an arrangement in which the eventual pattern formedon each part of the substrate is built up over a given period of time ora given number of exposures during which the pattern on the array ofindividually controllable elements and/or the relative position of thesubstrate changes.

Generally, the pattern created on the target portion of the substratewill correspond to a particular functional layer in a device beingcreated in the target portion, such as an integrated circuit or a flatpanel display (e.g., a color filter layer in a flat panel display or athin film transistor layer in a flat panel display).

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO, such as for example a mercury-arc lamp forproviding g-line or i-line UV radiation, or an excimer laser forproviding DUV radiation of a wavelength of less than about 270 nm, suchas for example about 248, 193, 157, and 126 nm. The radiation sourceprovides radiation having a wavelength of at least 5 nm, at least 10 nm,at least 11-13 nm, at least 50 nm, at least 100 nm, at least 150 nm, atleast 175 nm, at least 200 nm, at least 250 nm, at least 275 nm, atleast 300 nm, at least 325 nm, at least 350 nm, or at least 360 nm.Alternatively, the radiation provided by radiation source SO has awavelength of at most 450 nm, at most 425 nm, at most 375 nm, at most360 nm, at most 325 nm, at most 275 nm, at most 250 nm, at most 225 nm,at most 200 nm, or at most 175 nm. The radiation can have a wavelengthincluding 436 nm, 405 nm, 365 nm, 355 nm, 248 nm, 193 nm, 157 nm, and/or126 nm.

The source SO and the lithographic apparatus may be separate entities,for example when the source SO is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam B is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising, for example,suitable directing mirrors and/or a beam expander. In other cases thesource SO may be an integral part of the lithographic apparatus, forexample when the source SO is a mercury lamp. The source SO and theilluminator IL, together with the beam delivery system BD if required,may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD to adjust the angularintensity distribution of the radiation beam at the patterning devicelevel. Generally, at least the outer and/or inner radial extent(commonly referred to as σ-outer and σ-inner, respectively) of theintensity distribution in a pupil plane IPU of the illuminator can beadjusted. In addition, the illuminator IL can comprise various othercomponents, such as an integrator IN and a condenser CO. The illuminatorcan be used to condition the radiation beam to have a desired uniformityand intensity distribution in its cross-section at the patterning devicelevel. The illuminator IL, or an additional component associated withit, can also be arranged to divide the radiation beam into a pluralityof sub-beams that can, for example, each be associated with one or aplurality of the individually controllable elements of an array ofindividually controllable elements. A two-dimensional diffractiongrating can, for example, be used to divide the radiation beam intosub-beams. In the present description, the terms “beam of radiation” and“radiation beam” encompass, but are not limited to, the situation inwhich the beam is comprised of a plurality of such sub-beams ofradiation.

The radiation beam B is incident on or emitted from the patterningdevice (e.g., mask) MA, which is held on the support structure (e.g.,mask table) MT, and is modulated by the patterning device MA inaccordance with a pattern MP. Having traversed the patterning device MA,the radiation beam B passes through the projection system PS, whichfocuses the beam B onto a target portion C of the substrate W.

The projection system has a pupil PPU conjugate to the illuminator pupilIPU. Portions of radiation emanate from the intensity distribution atthe illuminator pupil IPU and traverse a patterning device patternwithout being affected by diffraction at a patterning device patterncreate an image of the intensity distribution at the illuminator pupilIPU.

With the aid of the second positioner PW and position sensor IF (e.g.,an interferometric device, linear encoder, or capacitive sensor), thesubstrate table WT can be moved accurately, e.g., so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g., after mechanical retrieval from a mask library, or during ascan.

In an example, movement of the support structure MT may be realized withthe aid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning), which form part of the first positioner PM.Similarly, movement of the substrate table WT may be realized using along-stroke module and a short-stroke module, which form part of thesecond positioner PW. In the case of a stepper (as opposed to a scanner)the support structure MT may be connected to a short-stroke actuatoronly, or may be fixed. It will be appreciated that the beam B canalternatively/additionally be moveable, while the object table and/orthe patterning device can have a fixed position to provide the requiredrelative movement. Such an arrangement can assist in limiting the sizeof the apparatus. As a further alternative, which can, e.g., beapplicable in the manufacture of flat panel displays, the position ofthe substrate table WT and the projection system PS can be fixed and thesubstrate W can be arranged to be moved relative to the substrate tableWT. For example, the substrate table WT can be provided with a system toscan the substrate W across it at a substantially constant velocity.

Patterning device MA and substrate W may be aligned using patterningdevice alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks P1, P2 as illustrated occupydedicated target portions, they may be located in spaces between targetportions (these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice MA, the patterning device alignment marks M1 and M2 may belocated between the dies.

The lithographic apparatus can be of a type having two (dual stage) ormore substrate and/or patterning devices tables. In such “multiplestage” machines, the additional tables can be used in parallel, orpreparatory steps can be carried out on one or more tables while one ormore other tables are being used for exposure.

The lithographic apparatus can also be of a type wherein at least aportion of the substrate can be covered by an “immersion liquid” havinga relatively high refractive index, e.g., water, so as to fill a spacebetween the projection system and the substrate. An immersion liquid canalso be applied to other spaces in the lithographic apparatus, forexample, between the patterning device and the projection system.Immersion techniques are well known in the art for increasing thenumerical aperture of projection systems. The term “immersion” as usedherein does not mean that a structure, such as a substrate, must besubmerged in liquid, but rather only means that liquid is locatedbetween the projection system and the substrate during exposure.

In one example, such as the embodiment depicted in FIG. 1, the substrateW has a substantially circular shape, optionally with a notch and/or aflattened edge along part of its perimeter. In another example, thesubstrate has a polygonal shape, e.g., a rectangular shape.

Examples where the substrate has a substantially circular shape includeexamples where the substrate has a diameter of at least 25 mm, at least50 mm, at least 75 mm, at least 100 mm, at least 125 mm, at least 150mm, at least 175 mm, at least 200 mm, at least 250 mm, or at least 300mm. Alternatively, the substrate has a diameter of at most 500 mm, atmost 400 mm, at most 350 mm, at most 300 mm, at most 250 mm, at most 200mm, at most 150 mm, at most 100 mm, or at most 75 mm,

Examples where the substrate is polygonal, e.g., rectangular, includeexamples where at least one side, at least 2 sides or at least 3 sides,of the substrate has a length of at least 5 cm, at least 25 cm, at least50 cm, at least 100 cm, at least 150 cm, at least 200 cm, or at least250 cm.

At least one side of the substrate has a length of at most 1000 cm, atmost 750 cm, at most 500 cm, at most 350 cm, at most 250 cm, at most 150cm, or at most 75 cm.

In one example, the substrate W is a wafer, for instance a semiconductorwafer. The wafer material can be selected from the group consisting ofSi, SiGe, SiGeC, SiC, Ge, GaAs, InP, and InAs. The substrate can be: aIII/V compound semiconductor wafer, a silicon wafer, a ceramicsubstrate, a glass substrate, or a plastic substrate. The substrate canbe transparent (for the naked human eye), colored, or absent a color.

The thickness of the substrate can vary and, to an extent, can depend onthe substrate material and/or the substrate dimensions. The thicknesscan be at least 50 μm, at least 100 μm, at least 200 μm, at least 300μm, at least 400 μm, at least 500 μm, or at least 600 μm. Alternatively,the thickness of the substrate can be at most 5000 μm, at most 3500 μm,at most 2500 μm, at most 1750 μm, at most 1250 μm, at most 1000 μm, atmost 800 μm, at most 600 μm, at most 500 μm, at most 400 μm, or at most300 μm.

The substrate referred to herein can be processed, before or afterexposure, in for example a track (a tool that typically applies a layerof resist to a substrate and develops the exposed resist), a metrologytool, and/or an inspection tool. In one example, a resist layer isprovided on the substrate.

It is to be appreciated that, although the description is directed tolithography, the patterned device MA can be formed in a display system(e.g., in a LCD television or projector), without departing from thescope of the present invention. Thus, the projected patterned beam canbe projected onto many different types of objects, substrates, displaydevices, etc.

The depicted apparatus of FIG. 1 could be used in at least one of thefollowing modes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-) magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In a pulse mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

4. Continuous scan mode is essentially the same as pulse mode exceptthat the substrate W is scanned relative to the modulated beam ofradiation B at a substantially constant speed and the pattern on thearray of individually controllable elements is updated as the beam Bscans across the substrate W and exposes it. A substantially constantradiation source or a pulsed radiation source, synchronized to theupdating of the pattern on the array of individually controllableelements, can be used.

5. In pixel grid imaging mode, which can be performed using thelithographic apparatus of FIG. 1, the pattern formed on substrate W isrealized by subsequent exposure of spots formed by a spot generator thatare directed onto patterning device MA. The exposed spots havesubstantially the same shape. On substrate W the spots are printed insubstantially a grid. In one example, the spot size is larger than apitch of a printed pixel grid, but much smaller than the exposure spotgrid. By varying intensity of the spots printed, a pattern is realized.In between the exposure flashes the intensity distribution over thespots is varied.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Example Scatterometery Apparatus

FIGS. 2A-2C schematically depict principles of angle-resolved andspectroscopic scatterometry according to an embodiment of the inventionby which one or more properties of the surface of a substrate W may bedetermined. In an embodiment, referring to FIG. 2A, the scatterometercomprises a radiation source 2 (e.g., a broadband (white light)radiation source), which directs radiation onto a substrate W. Thereflected radiation is passed to a sensor 4 (e.g., a spectrometerdetector) which measures a spectrum 10 (intensity as a function ofwavelength) of the specular reflected radiation. From this data, thestructure or profile giving rise to the detected spectrum may bereconstructed, e.g., by Rigorous Coupled Wave Analysis and non-linearregression or by comparison with a library of simulated spectra, asshown in FIGS. 2B and 2C. In general, for the reconstruction, thegeneral form of the structure is known and some parameters are assumedfrom knowledge of the process by which the structure was made, leavingonly a few parameters of the structure to be determined from thescatterometry data.

The scatterometer may be a normal-incidence scatterometer or anoblique-incidence scatterometer. Variants of scatterometry may also beused in which the reflection is measured at a range of angles of asingle wavelength, rather than the reflection at a single angle of arange of wavelengths.

FIG. 3 schematically depicts a scatterometer according to an embodimentof the present invention. Radiation emitted by a radiation source 2 iscollected by illumination system 11-14 and focused onto a spot coveringa target on substrate W via an objective 15 and partially reflectivemirror 16. The objective 15 has a high numerical aperture (NA), in anembodiment at least 0.9 or at least 0.95. An immersion scatterometer mayeven have a lens with a numerical aperture over 1. Radiation reflectedby the substrate then transmits through partially reflective surface 16into a detector 4 in order to have the scatter spectrum detected. Thedetector 4 is located in the back-projected pupil plane of the lens 15or the pupil plane may instead be re-imaged with auxiliary optics 17onto the detector 4. The pupil plane is the plane in which the radialposition of radiation defines the angle of incidence and the angularposition defines the azimuth angle of the radiation. The radiationsource 2 may be part of the scatterometer or may simply be conduit ofradiation from an outside radiation generator.

In an embodiment, the detector is a two-dimensional detector so that atwo-dimensional angular scatter spectrum of the substrate target can bemeasured. The detector 4 may be, for example, an array of CCD or CMOSsensors, and may have an integration time of, for example, 40milliseconds per frame.

The detector 4 may measure the intensity of scattered radiation at asingle wavelength (or narrow wavelength range), the intensity separatelyat multiple wavelengths or the intensity integrated over a wavelengthrange. Furthermore, the detector may separately measure the intensity oftransverse magnetic- and transverse electric-polarized radiation and/orthe phase difference between the transverse magnetic- and transverseelectric-polarized radiation.

Using a broadband radiation source (i.e. one with a wide range ofradiation frequencies or wavelengths—and therefore of colors) ispossible, which gives a large etendue, allowing the mixing of multiplewavelengths. Several “sources” of radiation may be different portions ofan extended radiation source which has been split using fiber bundles.In this way, angle resolved scatter spectra may be measured at multiplewavelengths in parallel. A 3-D spectrum (wavelength and two differentangles) may be measured, which contains more information than a 2-Dspectrum. This allows more information to be measured which increasesmetrology process robustness.

The target on substrate W is selected so that it is sensitive to aparameter of the lithographic process to be investigated, for example,focus, dose or overlay. It may be a grating which is printed such thatafter development, the bars are formed of solid resist lines. The barsmay alternatively be etched into the substrate. The scatterometry dataof the printed grating can used by processing unit PU to reconstruct thetarget to derive from it a value for the parameter under investigation.One or more parameters of the ideal target, such as line widths andshapes, may be input to the reconstruction process from knowledge of theprinting step and/or other scatterometry processes. Alternatively oradditionally, information indicative of the parameter underinvestigation my derived directly from the scatterometry data, e.g. by atechnique such as Principle Component Analysis.

The illumination system of the scatterometer SM can be regarded asformed of two parts: a first part, represented by lenses 11, 13, formsan intermediate image 2′ of the radiation source 2, while a second part,represented by lens 14, working with the high-NA lens 15 images theintermediate image 2′ onto the substrate W.

In an embodiment, an illumination aperture blade 12 is provided in thefirst part of the illumination system and is imaged into the back focalplane of the high-NA lens 15. The illumination aperture blade defines anillumination mode, for example annular illumination, suitable for theintended measurement, e.g. overlay. Because the illumination apertureblade 12 blocks some of the spatial frequencies of the source 2, theimage of the source on the substrate is broadened and radiation spillsoutside of the desired target area. Radiation reflected by structuresoutside the target area may cause noise in the scatterometry data.Therefore, a field stop 19 is provided at the intermediate image 2′ ofthe radiation source 2. The field stop 19 is desirably only slightlylarger than the ideal geometric spot width (e.g., diameter) andtherefore blocks radiation that is diffracted outside the geometricspot, ensuring the spot projected onto the substrate is as sharp aspossible.

It is noted that the field stop 19 also acts as a low-pass filter forthe image of the aperture blade 12 in the back focal plane of thehigh-NA lens 15, thus blurring the illumination mode. This may reducethe angular resolution of the diffraction orders on the detector 4.However, by using an apodized field stop, i.e. one in which thetransition from transparent to opaque is gradual rather than step wise,an optimum balance between stray radiation in the image plane andangular resolution of the diffraction orders can be obtained.

Catadioptric Optical Systems Used For Scatterometry

In an embodiment, the scatterometer includes a catadioptric opticalsystem.

The catadioptric optical system may be used as a special objective in aUV-visible scatterometer for critical dimension (CD) and overlaymeasurements as, for example, depicted in and described below withrespect to FIG. 4.

The catadioptric objective may be used together with the illuminationsystem of the scatterometer as, for example, depicted in and describedbelow with respect to FIG. 11.

In an embodiment, the catadioptric optical system may include (i) amirror system to provide a high numerical aperture and achromatism, and(ii) a nearly afocal refractive element to correct one or moreaberrations (such as coma).

A catadioptric optical system in a scatterometer can have severaldesirable characteristics. For example, a catadioptric optical systemcan have a very high numerical aperture (such as, for example,approximately 0.95) and operate in a wide spectral range (such as, forexample, approximately 193 nanometers to 1050 nanometers). In addition,a catadioptric optical system can produce low obscuration in the sensingbranch (approximately 15%). Moreover, a catadioptric optical system caninclude fewer optical surfaces in the sensing branch compared to aconventional scatterometer, thereby reducing or minimizing scatteringand ghost images produced in the sensing branch. Furthermore, acatadioptric optical system has smaller dimensions and weight comparedto conventional scatterometers.

An achromatic, high numerical aperture catadioptric optical system inaccordance with an embodiment of the present invention includes a convexspherical surface and a concave aspherical surface positioned to receiveelectromagnetic radiation from the convex spherical surface.

FIG. 4 depicts a scatterometry system 400 that can sense one or moreproperties of the surface of substrate 490. Scatterometry system 400 iscomparable to the one shown in FIG. 3. For example, elements 11, 13 and14 of FIG. 3 are comparable to elements 420, 422, 424, 432 of FIG. 4.Similarly, objective 15 of FIG. 3 may be or include catadioptric opticalsystem 480 of FIG. 4.

System 400 has an alignment branch and a sensing branch and includes acatadioptric optical system 480. In the embodiment depicted in FIG. 4,catadioptric optical system 480 includes a optical element 434 (e.g., abeam splitter) and an objective system 470. The alignment branch,sensing branch, and catadioptric optical system 480 are described inmore detail below.

The alignment branch is used to align system 400 with features on asubstrate 490. The alignment branch includes an illumination source 412(such as a wide band light emitting diode (LED)) that provides a firstbeam of radiation. In an example, the first beam has a spectral rangebetween 450 nanometers and 600 nanometers. The first beam passes throughoptical elements 430 and 432 and then impinges on an optical element434. The first beam is then directed through or adjacent objective 470and focused on a portion of substrate 490. The first beam is thenredirected (e.g., reflected and/or refracted) back through opticalelement 434 (e.g., via objective 470 if the first beam is directed tothe substrate via objective 470). A beam splitter 436 directs the firstbeam through a focusing lens 450 and beam splitter 452, and then onto afirst sensor 454 (e.g., a charge coupled device (CCD)). The image ofsubstrate 490, provided by sensor 454, is used to align system 400 withspecific portions of substrate 490.

The sensing branch is used to sense or detect the features on thealigned portions of substrate 490 according to known scatterometrytechniques, such as the scatterometry techniques described above. Thesensing branch includes an illumination source 410 (such as a tungstenillumination source having an interference filter) that provides asecond beam of radiation. In an example, the second beam has a bandwidthof approximately 10 nanometers and falls within the spectral range ofapproximately 300 nanometers to 800 nanometers. The second beam passesthrough optical elements 420, 422, 424, 430 and lens 432. Opticalelement 434 then directs the second beam through objective system 470and onto an aligned portion of substrate 490. The second beam isredirected by the aligned portion of substrate 490 and directed backthrough objective system 470 and optical element 434. The second beampasses through beam splitter 436, lens 440, aperture 442, and lens 444,and then impinges on a second detector 446 (e.g., second CCD). Seconddetector 446 provides an image of the aligned portion of substrate 490that is used to detect features on the surface of substrate 490.

As mentioned above, catadioptric optical system 480 includes opticalelement 434 and objective system 470. Catadioptric optical system 480 isachromatic in a wide spectral range (such as about 193 nanometers to1050 nanometers). When used in system 400, catadioptric optical system480 has low obscuration in the sensing branch (such as approximately 15%by radius). It has smaller dimensions and weight, and only a fewsurfaces thereby reducing scatter and eliminating ghost images. Whenused for sensing, objective system 470 can have a high numericalaperture (such as, for example, approximately 0.90 or 0.95) and does notuse refractive elements. As a result, objective system 470 operatesproperly over a wide spectral range (such as about 193 nanometers to1050 nanometers).

In an embodiment, the alignment branch and the sensing branch may bothshare catadioptric optical system 480. In an embodiment, catadioptricoptical system 480 functions properly within the optical specificationsof both the alignment branch and the sensing branch. In such anembodiment, the refractive elements of the alignment branch are situatedin a volume that is obscured by a small spherical mirror of thecatadioptric optical system 480. A first surface (or group of surfaces)in the alignment branch has a common surface (or surfaces) with a convexreflective surface in the sensing branch. The convex reflective surfacecan be partly reflective (such as, for example, 80% reflection) or havea spectral-dependent reflection that provides radiation distributionbetween the sensing and alignment branches. For example, the convexreflective surface can be conditioned (e.g., coated) to cause it to have(i) refractive properties for the electromagnetic radiation from thealignment branch and (ii) reflective properties for the electromagneticradiation from the sensing branch. Where used in the alignment branch insystem 400, catadioptric optical system 480 has substantially noobscuration in the alignment branch. Alternatively, the catadioptricoptical system 480 can be used in a system that only includes a sensingbranch.

Additional catadioptric optical systems in accordance with embodimentsof the present invention are described and illustrated below, forexample, in FIGS. 5-9. In each of the embodiments depicted in FIGS. 5-9,collimated electromagnetic radiation from an illumination system isfocused onto a small spot (such as approximately 10 microns) on asubstrate (e.g., a wafer). Each embodiment can be used forscatterometry, and each embodiment has an extremely wide numericalaperture (such as a numerical aperture of approximately 0.95) andoperates in a wide spectral range (such as about 193 nanometers to 1050nanometers).

FIG. 5 depicts an example catadioptric optical system 600 in accordancewith an embodiment. As shown in FIG. 5, catadioptric optical system 600includes a correcting plate 610, a spherical convex mirror 616, and anaspherical concave mirror 612.

Correcting plate 610 conditions a beam of radiation to correct one ormore optical aberrations (such as coma). As shown in FIG. 5, correctingplate 610 includes an aspherical surface s2 and a spherical surface s3.Radiation conditioned by correcting plate 610 passes through a hole 614in aspherical concave mirror 612 and impinges on spherical convex mirror616. Spherical convex mirror 616 comprises a spherical reflectivesurface s6 that is positioned to reflect the radiation conditioned bycorrecting plate 610. Aspherical concave mirror 612 receives theradiation reflected by spherical reflective surface s6. Asphericalconcave mirror 612 comprises an aspherical reflective surface s7 thatfocuses this radiation on a target portion of the substrate. Forexample, an example ray 611 reflected by aspherical reflective surfaces7 is depicted in FIG. 5.

FIG. 6 depicts an example catadioptric optical system 700 in accordancewith a further embodiment. As shown in FIG. 6, catadioptric opticalsystem 700 includes a correcting plate 710, a spherical convex mirror716, and a monolithic glass element 712.

Correcting plate 710 conditions a beam of electromagnetic radiation tocorrect one or more optical aberrations (such as coma). Correcting plate710 includes an aspherical surface s2.

Spherical convex mirror 716 comprises a spherical reflective surface s4that is positioned to reflect the electromagnetic radiation conditionedby correcting plate 710. In the embodiment depicted in FIG. 6, sphericalconvex mirror 716 is positioned on a surface s6 of monolithic glasselement 712. Aspheric surface s5 of monolithic glass element 712 has areflective portion and a transparent portion. Transparent portion iscentered around the optical axis and has a size based on thecross-sectional size of the input beam. As a result, surface s5 passes abeam coming from correcting plate 710, but reflects rays coming fromspherical mirror 716. That is, electromagnetic radiation conditioned bycorrecting plate 710 passes through the transparent portion of surfaces5 in monolithic glass element 712 and impinges on spherical convexmirror 716.

Monolithic glass element 712 includes surfaces s4, s5 and s6. Surface s5of monolithic glass element 712 receives the radiation reflected byspherical convex mirror 716 (surface s4) and reflects this radiationtoward a target portion of the substrate. Before impinging on the targetportion of the substrate, the radiation traverses surface s6 ofmonolithic glass element. Rays reflecting off of aspheric reflectivesurface s5 exit monolithic glass element 712 substantiallyperpendicularly to surface s6, and are therefore substantially notrefracted by surface s6. As a result, catadioptric optical system 700 isachromatic.

FIG. 7 depicts an example catadioptric optical system 800 in accordancewith a further embodiment. As shown in FIG. 7, catadioptric opticalsystem 800 includes a correcting plate 810, a spherical convex mirror816, an aspherical concave mirror 812, and a element 820.

Correcting plate 810 conditions a beam of radiation to correct one ormore optical aberrations (such as coma). Correcting plate 810 includesan aspherical surface s1 and a surface s2. As illustrated in FIG. 7,correcting plate 810 is positioned in a hole 814 of aspherical concavemirror 812.

Spherical convex mirror 816 comprises a spherical reflective surface s3that is positioned to reflect the radiation conditioned by correctingplate 810. In the embodiment depicted in FIG. 7, spherical convex mirror816 is positioned on a surface s5 of element 820. Radiation conditionedby correcting plate 810 impinges on spherical convex mirror 816.

Aspherical concave mirror 812 includes aspheric reflective surface s4.Aspherical reflective surface s4 of aspherical concave mirror 812receives the radiation reflected by spherical convex mirror 816 andreflects this radiation toward element 820 (e.g., a meniscus).

Element 820 includes a first surface s5 and a second surface s6. Theradiation reflected by aspherical concave mirror 812 passes throughelement 820 substantially perpendicularly to both first surface s5 andsecond surface s6, and is therefore substantially not refracted ateither surface of element 820. As a result, catadioptric optical system800 is achromatic.

FIG. 8 depicts an illumination system and catadioptric optical system900 in accordance with a further embodiment. Catadioptric optical system900 has an illumination numerical aperture of approximately 0.95 andworks in a wide spectral range from approximately 300 nanometers to 800nanometers. Catadioptric optical system 900 creates a small spot (suchas, for example, about a 10 micron spot) on a substrate 910.

Catadioptric optical system 900 includes a spherical refractive surface920, a plane reflective surface 930, an aspherical reflective surface940, an optical element 960, a group of lenses 970, a subsidiary lens980, and an illumination source 990 conjugate to the spot on substrate910. Illumination source 990 provides radiation that propagates throughsubsidiary lens 980 and lenses 970. Lenses 970 have at least oneaspheric surface. Lenses 970 function to correct aberrations (such ascoma) of catadioptric optical system 900. Lenses 970 may form an afocallens group 970. Optical element 960 directs the radiation from lenses970 to reflect off of plane reflective surface 930. The electromagneticradiation then reflects off of aspherical reflective surface 940, passesthrough spherical refractive surface 920, and is focused on substrate910. The radiation traverses spherical refractive surface 920 in adirection that is substantially perpendicular to surface 920. As aresult, catadioptric optical system 900 is achromatic.

Catadioptric optical system 900 can be used to test or sense features ofsubstrate 910. In the sensing mode, catadioptric optical system 900works as a high numerical aperture Fourier objective, wherein radiationpropagates in the opposite direction of that shown in FIG. 8.Specifically, radiation will be redirected by the surface of substrate910, traverse through catadioptric optical system 900, and impinge upona CCD located in a plane conjugate with the back focal plane ofcatadioptric optical system (i.e., the pupil plane). Radiation spotslocated at different points on the CCD correspond to beams of radiationredirected at different angles from the surface of substrate 910. Usingknown scatterometry techniques, these spots can be used to analyzefeatures of substrate 910 (such as CD and overlay).

For example, FIG. 9 depicts three redirected beams 913, 915, and 917(corresponding to redirected rays at about 0, 30, and 72 degrees fromthe surface of substrate 910) propagating through catadioptric opticalsystem 900 in the sensing mode. The redirected beams create a Fourierpattern in the pupil plane of catadioptric optical system 900.

Aspherical mirror 940, plane mirror 930, and refractive sphericalsurface 920 can be made from a monolithic glass optical element 912.Monolithic glass optical element 912 can be fabricated from a glass (forexample, SiO₂) that transmits in a spectral range from approximately 193nanometers to 1050 nanometers. In this example, plane mirror 930comprises an annulus with refractive spherical surface 920 positioned inthe center of the annulus. Monolithic glass optical element 912 isoriented to cause an illumination spot on substrate 910 to be concentricwith plane mirror 930 and refractive spherical surface 920. Opticalelement 960 can be fabricated from the same material as monolithic glasselement 912 and assembled by optically contacting it with monolithicglass element 912.

FIG. 10 depicts an example catadioptric optical system 1200 inaccordance with a further embodiment. Catadioptric optical system 1200includes a first monolithic glass element 1210, a second monolithicglass element 1220, and a refractive lens group 1230 cascaded together.Monolithic glass element 1210 transitions from a numerical aperture ofapproximately 0.95 to numerical aperture of approximately 0.4 (andback). Cascading monolithic glass elements 1210 and 1220 transitionsfrom a numerical aperture of approximately 0.95 to a numerical apertureof approximately 0.02.

First monolithic glass element 1210 includes a refractive surface s2, anaspherical reflective surface s3, a plane reflective surface s4, and arefractive surface s5. As illustrated in FIG. 10, refractive surface s2is positioned in the center of plane reflective surface s4, andrefractive surface s5 is positioned in the center of asphericalreflective surface s3.

Second monolithic glass element 1220 includes a reflective surface s7and a reflective surface s8 surfaces s7 and s8 each include a central,transparent portion.

Refractive lens group 1230 includes optical surfaces s9, s10, s11, s12,s13, and s14, which are positioned and shaped to correct one or moreaberrations (such as coma).

This optical design functions similar to the designs depicted in FIGS. 8and 9, but has just one aspheric surface (aspherical reflective surfaces3 of first monolithic glass element 1210) and a wider spectral range(about 193 to 1050 nanometers).

For example, radiation enters catadioptric optical system 1200 throughrefractive lens group 1230. The radiation passes through refractive lensgroup 1230, and then through the central, transparent portion ofreflective surface s8.

The radiation passing through the central, transparent portion ofreflective surface s8 is reflected by reflective surface s7 and thenreceived by reflective surface s8. Reflective surface s8 focuses theradiation into a focused spot of radiation that passes through thecentral, transparent portion of reflective surface s7. That is, secondmonolithic glass element 1220 is configured to provide a focused spot ofradiation.

Refractive surface s5 of first monolithic glass element 1210 ispositioned to be concentric with the focused spot of radiation fromsecond monolithic glass element 1220. Consequently, radiation fromsecond monolithic glass element 1220 enters first monolithic glasselement 1210 substantially perpendicularly to refractive surface s5.Reflective surface s4 receives this radiation and reflects it towardaspherical reflective surface s3. Aspherical reflective surface s3focuses the radiation onto a focused spot on a substrate (notspecifically shown in FIG. 10). Refractive surface s2 is positioned tobe concentric to the focused spot on the substrate, thereby causing theradiation to exit first monolithic glass element 1210 substantiallyperpendicular to refractive surface s2.

Because radiation enters and exits first monolithic glass element 1210substantially perpendicularly to refractive surfaces s5 and s2,catadioptric optical system is substantially achromatic, i.e., having aspectral range of approximately 193 to 1050 nanometers.

A metrology tool, such as a scatterometer as discussed above, mayoperate in wide spectral range (e.g., approximately 193 to 1050nanometers) with a high numerical aperture. However, such a metrologytool may have effectively a shorter spectral band (e.g., 300-800 nm)because the illumination system for the metrology tool is unable toprovide the wide spectral range. This is due to the chromatic limitationof refractive elements in the illumination system.

FIG. 11 shows a schematic view of a scatterometer according to anembodiment of the invention. FIG. 11 essentially shows the illuminationsystem of the scatterometer which supplies radiation to the objective 15(which as discussed above may be, or include, a catadiopric opticalsystem). Referring to FIG. 11, the illumination system includes anillumination fiber 1300 to bring radiation from an optical source 2 (notshown) to the illumination system. The illumination system furtherincludes a condenser lens 11 and relay lenses 13 and 14.

To enable the illumination system to provide a wide spectral range, anachromatic condenser lens 11 with input NA>0.05 that can effectivelycollect light from high NA illumination fiber 1300 should be provided.

According to an embodiment of the invention, a catadioptric opticalsystem is provided as condenser lens 11, hereinafter the catadioptriccondenser lens. It performs the function of a condenser lens but isachromatic and operates in wide spectral range from about 193 to 1050nm, Design parameters of the catadioptric condenser lens can be selectedin order to have minimal central obscuration. In an embodiment, theobscuration of the catadioptric condenser lens may match that of theobjective 15. For example, the obscuration may be less than or equal tothat in the objective 15. In that case the catadioptric condenser lenswould not introduce additional losses of radiation in the system thanthat would be provided by the objective 15. In an embodiment, theobscuration may be up to 20% more than that of the objective. In anembodiment, the obscuration is approximately 15% or less by pupilradius. Relay lenses 13 and 14 may be fabricated as doublets from CaF₂and/or SiO₂ and provide matching of illumination field size downstreamof the catadioptric condenser lens and the size of the pupil of theobjective.

FIG. 12 schematically depicts a catadioptric condenser lens 11 accordingto an embodiment of the invention. The catadioptric condenser lens 11includes a first transparent material block 1310 and a secondtransparent material block 1320. The first block 1310 and the secondblock 1320 may be separate optical parts fabricated into a single pieceat an interface 1340 or may be a monolithic element of transparentmaterial. The material of the first block 1310 and/or the second block1320 may be fused silica (SiO₂), calcium fluoride (CaF₂) or glass.

The first block 1310 comprises a spherical mirror 1350. In anembodiment, the first block 1310 has an output surface 1360 (e.g., aflat surface) in which there is a cavity that forms the spherical mirror1350. The cavity may be, e.g., silvered. In an embodiment, the cavitymay be filled with a material. The material in the cavity may bedifferent than that of the first block 1310, in which case silvering maynot be necessary.

The second block 1320 comprises a reflective surface 1370. In anembodiment, the reflective surface 1370 is aspherical. In an embodiment,the reflective surface 1370 may be embedded in the second block 1320 asshown. This may be produced, for example, by contacting a silvered firstpart of second block 1320 with a conforming second part of second block1320 having surface 1390. The second part of second block 1320 may be ofa different material than that of the first part of second block 1320,in which case silvering may not be necessary. In an embodiment, thesurface 1390 may be the reflective surface 1370.

The second block 1320 further comprises a hole 1380 extending into thesecond block 1320 from the surface 1390 of the second block 1320. In anembodiment, the hole is cylindrical.

Optical fiber 1300 extends into the hole 1380 of the second block 1320and terminates at outlet 1330 of the optical fiber 1300. In anembodiment, the outlet 1330 ends at the interface 1340 between the firstblock 1310 and the second block 1320. The outlet 1330 of illuminationfiber 1300 acts as an input illumination source. In an embodiment,optical fiber 1300 comprises a cylindrical hollow pipe with a reflectiveinternal surface to deliver illumination radiation. In an embodiment,the outlet 1330 is between the mirror 1350 and the reflective surface1370 and in an embodiment, about midway or closer to the mirror 1350.

Radiation from the outlet 1330 is reflected from mirror 1350, propagatesthrough all or part of the first block 1310 and the second block 1320,and then reflects from reflective surface 1370 to create a collimatedbeam of radiation 1400 passing through the output surface 1360. Becausethe beam is reflected around the mirror 1350, the beam 1400 has acentral obscuration. In an embodiment, output surface 1360 issubstantially perpendicular to the direction of the beam from thereflective surface 1370. This facilitates an achromatic beam. Forexample, output surface 1360 is flat.

An example optical prescription of the catadioptric condenser lens ispresented in Table 1 below.

TABLE 1 Example optical prescription for a catadioptric condenser lens -reflective surface 1370 is conical with K = −0.8431, the diameter ofoptical fiber 1300 is assumed to be 200 μm, the material of thecatadioptric condenser lens is UV fused silica, and the output beam 1400diameter is 4.8 mm. Surface Y Semi- Surface # Type Y Radius ThicknessGlass Mode Aperture Object Sphere Infinity Infinity Refract Stop SphereInfinity 10.0311 Silica Refract 2.4000 2 Conic −23.0688 −10.0311 SilicaReflect 2.4323 3 Sphere −4.8156 4.0000 Silica Reflect 0.3497 4 SphereInfinity 0.0000 Refract 0.1022 Image Sphere Infinity 0.0000 Refract0.1022

The design shown and described in FIG. 12 and Table 1 has a centralobscuration of about ˜15 % to match the obscuration in the objective 15.Design parameters can be scaled or modified to meet packaging needs of aspecific metrology tool (e.g., depending on fiber diameter, objectivepupil size, magnification of the relay system, etc.).

In an embodiment, a space between the reflective surface 1350 and thereflective surface 1370 and at least within an outer lateral boundary ofreflective surface 1350 or reflective surface 1350 may be filled by gas(e.g., air or nitrogen) or liquid and illumination fiber 1300 isimbedded inside of a hollow mounting pipe surrounded by the gas orliquid. The pipe is connected to the center of reflective surface 1370and extends at least into the gas or liquid between reflective surface1350 and reflective surface 1370. In a variant, the mounting pipe mayhave a reflective internal surface and the illumination fiber 1300terminates at the entrance of the pipe, as similarly described belowwith respect to FIG. 13. The radiation would enter the pipe, reflectfrom the internal surface, and output from outlet 1330.

In an embodiment, referring to FIG. 13 which is similar in many respectsto FIG. 12, the optical fiber 1300 is removed from the hole 1380, whichis filled with an optical medium (e.g., a transparent material such asglass or a liquid) with a higher refractive index than the material insecond block 1320. The outlet 1330 is adjacent to or contacts theradiation exit of the hole 1380. Thus, the hole 1380 acts as a waveguide to deliver illumination radiation to point 1410.

Conclusion

Catadioptric optical systems for a metrology tool, such as ascatterometer, have been described. While various embodiments of thepresent invention have been described above, it should be understoodthat they have been presented by way of example only, and notlimitation. It will be apparent to persons skilled in the relevant artthat various changes in form and detail can be made therein withoutdeparting from the spirit and scope of the invention. For example, oneor more features of an embodiment described herein may combined with orreplace one or more features of another embodiment described herein. Forexample, a catadioptric optical system described above for the objective15 may be used in the illumination system if suitable modified to outputa collimated beam.

A person skilled in the relevant art(s) can modify and re-optimize theabove-described embodiments to better comply with a fabrication processof or optics included in the sensing and the alignment branches. Forexample, convex spherical mirrors 616, 716, 816, and 1350 (of FIGS. 5,6, 7 and 12, respectively) can be replaced by concave or asphericalmirrors. These, and other modifications, of the above-describedembodiments will become apparent to a person skilled in the relevantart(s), and are intended to be within the spirit and scope of thepresent invention.

In an embodiment, there is provided a metrology tool, comprising: anobjective to deliver radiation to a surface and to receive radiationredirected by the surface; a detector to receive the redirectedradiation from the objective; and an illumination system to deliver theradiation for redirection to the objective, the illumination systemcomprising an illumination system catadioptric optical system.

In an embodiment, the objective comprises an objective catadioptricoptical system. In an embodiment, the metrology tool is a scatterometer.In an embodiment, the illumination system catadioptric optical system,comprises: a first reflective surface positioned and configured toreflect radiation; a second reflective surface positioned and configuredto reflect radiation reflected from the first reflective surface as acollimated beam, the second reflective surface having an aperture toallow transmission of radiation through the second reflective surface;and a channel structure extending from the aperture toward the firstreflective surface and having an outlet, between the first reflectivesurface and the second reflective surface, to supply radiation to thefirst reflective surface. In an embodiment, the second reflectivesurface forms or is on a surface of a first solid transmissive elementpart and the channel structure comprises a channel aperture through thefirst solid transmissive element part. In an embodiment, the metrologytool is configured to transmit all wavelengths of radiation from therange of approximately 193 to 1050 nanometers. In an embodiment, themetrology tool is configured to obscure approximately 15% or less of theradiation by pupil radius.

In an embodiment, there is provided a catadioptric optical system,comprising: a first reflective surface positioned and configured toreflect radiation; a second reflective surface positioned and configuredto reflect radiation reflected from the first reflective surface as acollimated beam, the second reflective surface having an aperture toallow transmission of radiation through the second reflective surface;and a channel structure extending from the aperture toward the firstreflective surface and having an outlet, between the first reflectivesurface and the second reflective surface, to supply radiation to thefirst reflective surface.

In an embodiment, the second reflective surface forms or is on a surfaceof a first solid transmissive element part. In an embodiment, the firstreflective surface forms or is on a surface of a second solidtransmissive element part adjacent the first solid transmissive elementpart. In an embodiment, the channel structure comprises a channelaperture through the first solid transmissive element part. In anembodiment, the outlet is at an interface between the first solidtransmissive element part and the second solid transmissive elementpart. In an embodiment, the first and second solid transmissive elementparts are portions of a monolithic solid transmissive element. In anembodiment, a fluid surrounds the channel structure, extends between thefirst and second reflective surfaces, and is located at least within anouter lateral boundary of the first reflective surface or the secondreflective surface. In an embodiment, the catadioptric optical systemfurther comprises an optical fiber extending in or forming the channelstructure. In an embodiment, the first reflective surface comprises aconvex reflective surface. In an embodiment, the beam from the secondreflective surface passes through a surface substantially perpendicularto the beam direction.

In an embodiment, there is provided a metrology method, comprising:delivering radiation to a surface using an objective; receivingradiation redirected by the surface using the objective; detecting aparameter of the surface using the redirected radiation from theobjective; and delivering the radiation for redirection to the objectiveusing a catadioptric optical system.

In an embodiment, the objective comprises a catadioptric optical system.In an embodiment, the objective is part of a scatterometer. In anembodiment, delivering the radiation comprises: reflecting radiationusing a first reflective surface of the catadioptic optical system; andreflecting radiation reflected from the first reflective surface using asecond reflective surface to form a collimated beam, the secondreflective surface having an aperture to allow transmission of radiationthrough the second reflective surface, wherein a channel structureextends from the aperture toward the first reflective surface and has anoutlet, between the first reflective surface and the second reflectivesurface, to supply radiation to the first reflective surface.

Furthermore, it is to be appreciated that the description may set forthone or more but not all exemplary embodiments of the present inventionas contemplated by the inventor(s), and thus, is not intended to limitthe present invention and the appended claims in any way. Thus, thebreadth and scope of the present invention should not be limited by anyof the above-described exemplary embodiments, but should be defined onlyin accordance with the following claims and their equivalents.

1.-20. (canceled)
 21. A metrology tool, comprising: a metrologyobjective configured to deliver metrology radiation to a surface and toreceive metrology radiation redirected by the surface; a detectorconfigured to receive the redirected metrology radiation from theobjective to facilitate a metrology measurement; and an illuminationsystem configured to deliver the metrology radiation for redirection tothe metrology objective, the illumination system comprising anillumination system catadioptric optical system.
 22. The metrology toolof claim 21, wherein the metrology objective comprises an objectivecatadioptric optical system.
 23. The metrology tool of claim 21, whereinthe metrology tool is a scatterometer.
 24. The metrology tool of claim21, wherein the illumination system catadioptric optical system isconfigured to output a substantially collimated beam of the metrologyradiation toward the metrology objective.
 25. The metrology tool ofclaim 21, wherein the illumination system catadioptric optical system,comprises: a first reflective surface positioned and configured toreflect metrology radiation; a second reflective surface positioned andconfigured to reflect metrology radiation reflected from the firstreflective surface as a substantially collimated beam, the secondreflective surface having an aperture to allow transmission of metrologyradiation through the second reflective surface; and a channel structureextending from the aperture toward the first reflective surface andhaving an outlet, between the first reflective surface and the secondreflective surface, to supply metrology radiation to the firstreflective surface.
 26. The metrology tool of claim 25, wherein thesecond reflective surface forms or is on a surface of a first solidtransmissive element part.
 27. The metrology tool of claim 26, whereinthe channel structure comprises a channel aperture through the firstsolid transmissive element part.
 28. The metrology tool of claim 26,wherein the first reflective surface forms or is on a surface of asecond solid transmissive element part adjacent the first solidtransmissive element part.
 29. The metrology tool of claim 28, whereinthe outlet is at an interface between the first solid transmissiveelement part and the second solid transmissive element part.
 30. Themetrology tool of claim 28, wherein the first and second solidtransmissive element parts are portions of a monolithic solidtransmissive element.
 31. The metrology tool of claim 25, wherein afluid surrounds the channel structure, extends between the first andsecond reflective surfaces, and is located at least within an outerlateral boundary of the first reflective surface or the secondreflective surface.
 32. The metrology tool of claim 25, furthercomprising an optical fiber extending in or forming the channelstructure.
 33. The metrology tool of claim 25, wherein the firstreflective surface comprises a convex reflective surface.
 34. Themetrology tool of claim 25, wherein the beam from the second reflectivesurface passes through a surface substantially perpendicular to the beamdirection.
 35. The metrology tool of claim 21, wherein the metrologyobjective and/or the illumination system are configured to transmit allradiation wavelengths from the range of approximately 193 to 1050nanometers.
 36. The metrology tool of claim 21, wherein the illuminationsystem is configured to output metrology radiation that has itscross-section obscured approximately 15% or less.
 37. A metrologymethod, comprising: delivering metrology radiation to a surface using ametrology tool objective; receiving metrology radiation redirected bythe surface using the metrology tool objective; detecting a parameter ofthe surface using the redirected metrology radiation from the metrologytool objective; and delivering the metrology radiation for redirectionto the metrology tool objective using a catadioptric optical system. 38.The metrology method of claim 37, wherein the catadioptric opticalsystem outputs a substantially collimated beam of the metrologyradiation towards the metrology objective.
 39. The metrology method ofclaim 37, wherein the metrology tool objective comprises a catadioptricoptical system.
 40. The metrology method of claim 37, wherein deliveringthe metrology radiation comprises: reflecting metrology radiation usinga first reflective surface of the catadioptric optical system; andreflecting metrology radiation reflected from the first reflectivesurface using a second reflective surface to form a substantiallycollimated beam, the second reflective surface having an aperture toallow transmission of metrology radiation through the second reflectivesurface, wherein a channel structure extends from the aperture towardthe first reflective surface and has an outlet, between the firstreflective surface and the second reflective surface, to supplymetrology radiation to the first reflective surface.